Rain recharges soil water storages and either percolates
downward into aquifers and streams or is returned to the atmosphere through
evapotranspiration. Although it is commonly assumed that summer rainfall
recharges plant-available water during the growing season, the seasonal
origins of water used by plants have not been systematically explored. We
characterize the seasonal origins of waters in soils and trees by comparing
their midsummer isotopic signatures (δ2H) to seasonal isotopic
cycles in precipitation, using a new seasonal origin index. Across 182 Swiss
forest sites, xylem water isotopic signatures show that summer rain was not
the predominant water source for midsummer transpiration in any of the three
sampled tree species. Beech and oak mostly used winter precipitation, whereas
spruce used water of more diverse seasonal origins. Even in the same plots,
beech consistently used more winter precipitation than spruce, demonstrating
consistent niche partitioning in the rhizosphere. All three species' xylem
water isotopes indicate that trees used more winter precipitation in drier
regions, potentially mitigating their vulnerability to summer droughts. The
widespread occurrence of winter isotopic signatures in midsummer xylem
implies that growing-season rainfall may have minimally recharged the soil
water storages that supply tree growth, even across diverse humid climates
(690–2068 mm annual precipitation). These results challenge common
assumptions concerning how water flows through soils and is accessed by
trees. Beyond these ecological and hydrological implications, our findings
also imply that stable isotopes of δ18O and δ2H in plant
tissues, which are often used in climate reconstructions, may not reflect
water from growing-season climates.

Plant water availability shapes ecosystems, climates, and natural resources.
In hydrology and ecology, soil water storage is often represented as a
bucket or vertical stack of well-mixed reservoirs, filled by previous
precipitation events, and used by plants as a function of their rooting
depth (Lawrence, et al., 2011; Wigmosta et al., 1994). The reality is more
complex: water transport through soils tends to be dominated by preferential
flow through large pores, whereas water is often primarily stored in the
finer matrix (Beven and Germann, 1982; Lawes et al., 1882). Thus, plant
water availability depends on the interplay between macropore flow, matrix
storage, and the rooting architecture of vegetation (Brooks et al., 2010;
Stewart et al., 1999; Tinker, 1976). Previous research has documented depths
of roots and root water uptake (Dubbert et al., 2019; Fan et al., 2017;
West et al., 2012), but little attention has been directed towards
understanding how water becomes available for uptake at those depths.

Water stable isotope signatures (δ2H and δ18O)
have been used as tracers to show that plant water uptake is not sourced
from the same subsurface storage as streamflow (Evaristo et al., 2015; Good
et al., 2015; Javaux et al., 2016), but it remains unclear how that storage
is replenished and becomes available to plants. Soils may retain a mixture
of waters that originate from many previous precipitation events (Botter et
al., 2011; Mueller et al., 2014; Sprenger et al., 2016b; Brinkmann et al.,
2018), but plants may not evenly sample from that distribution of water
ages, because plants may root such that they preferentially take up water
moving along faster or slower pathways (Brooks et al., 2010; Ehleringer et
al., 1991; Stewart et al., 1999). These interactions between root
distributions and infiltration dynamics could hypothetically result in
plants disproportionally using precipitation from past seasons rather than
recent precipitation. While a few case studies have reported plants
predominantly using precipitation from past seasons in arid (Ehleringer et
al., 1991) or Mediterranean climates (Brooks et al., 2010; Rempe and
Dietrich, 2018) where there is minimal growing-season precipitation, the
seasonal origins of water used by plants have not been systematically
explored in humid climates.

To investigate the seasonal origins of waters that supply midsummer tree
growth, we analyzed xylem water isotopes from a snapshot sample of 918 trees
from three dominant species in 182 forest sites across Switzerland. At 31 of
these sites, we complemented the xylem water with isotope values of soil
waters, sampled using suction lysimeters (which are generally considered to
access the more mobile fraction of soil waters that are not held under high
tensions; Brooks et al., 2010). To characterize the seasonal origins of
xylem water and lysimeter soil water, we developed a seasonal origin index,
based on the isotopic signature of soil and plant water relative to the seasonal
precipitation isotope cycle; this index quantifies the overexpression of
winter versus summer (recent) precipitation in xylem or lysimeter waters,
relative to annual precipitation. This new seasonal origin index can be
effectively used in these sites because the strong seasonal isotopic cycle
in Swiss precipitation (Allen et al., 2018) allows for winter and summer
precipitation to be clearly distinguished in tree xylem. We used this
midsummer snapshot to determine (a) whether summer or winter precipitation
was overrepresented in midsummer soil and xylem waters, relative to annual
precipitation; (b) how the seasonal origins of xylem water varied across
diverse climates and site characteristics; and (c) whether these three
dominant trees species differed in their water sources.

where δx is the fractionation-compensated δ2H
isotopic signature of xylem water or lysimeter soil water, and δwinterP, δsummerP, and δannP are
the δ2H isotopic signatures of typical winter, typical
summer, and volume-weighted annual precipitation at each study site (see
Sect. 2.4 and Fig. 1). This index expresses the isotopic signature of soil
and plant water relative to seasonal precipitation isotope cycles, which are
especially strong in high-latitude, continental interiors, where
precipitation isotopes are heavy in summer and light in winter (Halder et
al., 2015; Vachon et al., 2007). The SOI will be near −1.0 for soil and
plant water samples derived entirely from winter precipitation and near 1.0
for samples derived entirely from summer precipitation (Fig. 1). Samples with
SOI values near zero approximate the annual average precipitation and can
potentially represent many possible mixtures of waters from spring, summer,
autumn, and winter. By extracting waters from tree xylem, which reflect the
waters taken up by roots (Newberry et al., 2017), and comparing those data to
precipitation isotopes, this metric is robust to several uncertainties that
are prevalent in isotope-based rooting depth studies, such as sampling and
extracting soil waters that are representative of the waters accessed by
roots, as described below in greater detail (Goldsmith et al., 2019; Orlowski
et al., 2018; Penna et al., 2018).

Figure 1Calculation of the seasonal origin index (SOI). As a hypothetical
example, consider one site that receives equal precipitation amounts
throughout the year and another site that receives more precipitation in
winter (a), but both have the same seasonal isotopic cycle (b). In this
example, the volume-weighted average precipitation is −75 ‰ δ2H in the uneven-precipitation site and
−55 ‰ δ2H in the even-precipitation site
(c); these values mark SOI = 0. Thus, if water with −60 ‰ δ2H was observed in xylem in the
uneven-precipitation site, SOI would be positive, indicating that each millimeter of
rain that fell during the summer made a larger contribution to xylem water
than each millimeter of rain that fell during the winter (even though, owing to the
greater precipitation in winter, winter precipitation made up more than
50 % of the xylem water). Panel (b) also shows how the seasonal
precipitation isotope cycle is defined by a fitted sinusoid, such that the
amplitude captures typical summer and winter peaks and not the absolute
bounds of possible values (i.e., SOI of soil or xylem water can be higher
than 1.0 or lower than −1.0).

In using this SOI, we implicitly test the null hypothesis that xylem and
lysimeter soil water are the annual volume-weighted average of
precipitation, and thus we center the SOI index such that SOI = 0 at that
value, in any precipitation regime (Fig. 1). We address the following question: is
winter or summer water overrepresented in soils or xylem, relative to
volume-weighted precipitation? Importantly, this SOI equation (Eq. 1)
differs from a simple, two-end-member (δwinterP and δsummerP) mixing model, which addresses a different question – is
there more winter water than summer water in soils or xylem? – but does
not account for the fact that we should expect more winter precipitation in
soils (for example) at sites with more winter rainfall. Thus the piecewise
linear equation that we use to define SOI is more appropriate for
determining whether winter or summer water is overrepresented in soils and
xylem (relative to precipitation inputs) across sites with different
seasonal patterns in precipitation; nonetheless, the two approaches yield
similar values in areas with relatively even precipitation throughout the
year, such as Switzerland (Fig. S1).

2.2 Field sites and measurements

The study was carried out in summer of 2015 at 182 sites established across
Switzerland as part of a forest health monitoring program (Braun et al.,
1999, 2017). Each site contained at least one of three tree species: 97
contained beech (Fagus sylvatica L.), 71 contained spruce (Picea abies (L.) H. Karst.), and 49 contained oak
(Quercus robur L.). Sites ranged from 255 to 1840 m a.s.l. in elevation,
3.3 to 11.1 ∘C in mean annual temperature, and 690 to 2068 mm in total annual
precipitation. The mean elevations of sites with oak (513 m a.s.l.) and
beech (617 m a.s.l.) were slightly lower than those of sites with spruce
(893 m a.s.l.). On average, mean annual precipitation at sites with oak
(1085 mm yr−1) was slightly lower than at sites with beech (1285 mm yr−1)
or spruce (1339 mm yr−1). Tree diameters, measured in 2014,
ranged from 17 to 105 cm. All stands are actively managed and composed of
mature trees in established, closed-canopy stands. Soils are highly variable
in depth (ranging from 30 to 220 cm) and texture (ranging from 4 % to 61 %
clays and 6 % to 81 % sands in the top 50 cm) across the sites (see Figs. S2
and S3). These soils have traits, profiles, and parent materials (documented
in the Supplement) that result in them spanning many types and
orders, but analyzing those classifications goes beyond the scope of this
paper.

To determine the δ18O and δ2H ratios of xylem
water in trees, branches were sampled from 3 to 8 individual trees of each of
the species present in each plot. All branch samples were collected between
27 July and 10 August 2015, using pole pruners operated by technicians
suspended below helicopters. Thus, all sampled trees occupied at least
intermediate canopy positions. On the ground, bark and phloem tissue was
removed from fully suberized branches, and samples were sealed in vials and
frozen for later extraction and isotopic analysis.

We also determine the δ18O and δ2H ratios of soil
waters accessed by suction lysimeters, which tend to sample a more mobile
fraction of soil water (i.e., in contrast to water under high tension or in
tight, low-conductivity pore spaces). These lysimeter soil water samples
were collected using porous suction cups (Soilmoisture Equipment Corp.,
Santa Barbara, USA) at 31 of the forest monitoring sites in July 2015. A
tension of 60–70 kPa was applied to each suction lysimeter once, within
1 month prior to the xylem sample collection date. Water samples were
collected from lysimeters where water could be extracted (i.e., a tension
could be applied without losing suction), approximately 4 to 5 weeks
after the tension was applied. This so-called “continuous mode” operation
is considered to sample flowing waters (Weihermüller et al., 2005),
although the actual extraction interval of these lysimeters was likely much
shorter than the entire 4 to 5 weeks. Each site had sets of suction
lysimeters at one to four different depths (often at 20, 50, and 80 cm, but
up to 120 cm deep; see Fig. S2) depending on the soil thickness. For each
depth and at each of the 31 sites, there were three to eight replicate
lysimeter sets (mean of 6.7). Replicate samples were pooled by depth, then
sealed and frozen in 50 mL vials for later isotopic analysis. We used these
lysimeter data to understand the seasonal origins of the more mobile
fractions of water in soils (Brooks et al., 2010; Sprenger et al., 2016a),
and we do not assume that they are representative of the entire soil water
pool, or the pool of water available to plants. While the applied tensions
act on all pores, we assume these samples to be more sourced from pores that
can conduct water more quickly to a lysimeter, although there is not an
explicit pore-size threshold (sensu Grossmann and Udluft, 1991).

To determine how the δ18O and δ2H ratios of xylem
water and lysimeter soil water varied as a function of soils and climate,
additional site metrics were measured. Soil texture (sand, silt, clay,
stone, and organic matter content) and fine root density by horizon
(including O horizons) were determined from a soil pit excavated at each
site to characterize soil properties. Measurement protocols were consistent
with the German soil survey (German BGR, 2005). Elevation was determined for
each site from a digital elevation model (25 m resolution; Swiss Federal
Office of Topography, Wabern, Switzerland). Slope and aspect were surveyed
at each field site, using a compass and clinometer. Mean temperature,
precipitation amount, and potential evapotranspiration (PET) were determined
using a geospatial model (Meteotest, Bern, Switzerland) based on weather
station data. To interpret the fine root density indices assigned to each
soil horizon, we converted the ordinal density indices assigned in the field
soil survey (W1, W2, W3, W4, W5, W6; German BGR, 2005) to the respective
mean values of the index categories (1.5, 4, 8, 15.5, 35.5, and 50 roots cm−2); the density-weighted
mean fine root depth (i.e., depth to center
of mass) was then calculated using those values for each site. The root
profiles, showing density by horizon, are provided in Fig. 2. The
Supplement includes taxonomic characterizations of the horizons
(from which soil types can be inferred) and by-horizon data on soil textures
and root densities.

Figure 2Isotope ratios of xylem water and lysimeter soil water, compared to
site-specific seasonal isotope cycles in precipitation.
Fractionation-compensated isotope ratios for xylem and soil lysimeter water
are plotted as deviations from each site's volume-weighted annual
precipitation δ2H for the two years prior to the summer 2015
sampling. Typical isotope values for summer and winter precipitation are
shaded (see methods). Magenta bars show the δ2H range of
lysimeter soil water in depth profiles. Sites are ranked by mean annual
precipitation amount (see dotted black line and labels below the horizontal
axis). The panel on the right depicts how isotope values translate to
seasonal origin index values. Trees in all but the wettest sites mostly use
water that isotopically resembles winter precipitation (i.e., negative SOI).

2.3 Sample processing and laboratory analyses

Water was extracted from branch xylem material by cryogenic vacuum
distillation (West et al., 2006) at the Paul Scherrer Institute (Villigen,
Switzerland). All samples were heated for 2 h to ensure complete
extraction. The δ18O and δ2H ratios of soil and
xylem water were subsequently analyzed using a high-temperature-conversion
elemental analyzer (TC/EA) connected to a Delta Plus XP isotope
ratio mass spectrometer via a Conflo III interface (Thermo Fisher
Scientific, Bremen, Germany). Isotope ratios are expressed in per mil
(‰) notation relative to V-SMOW (Vienna Standard Mean Ocean Water).
The long-term instrument precision, measured using independent quality control standards,
is ≤0.4 ‰ for δ2H and ≤0.2 ‰ for δ18O. There has been considerable
debate over cryogenic vacuum extraction because studies have observed
discrepancies in cryogenic extraction of soil water in rehydration
experiments (Meißner et al., 2014; Oerter et al., 2014; Orlowski et al.,
2018); however, soil waters were not cryogenically extracted in this study,
and those discrepancies are not observed when extracting xylem water
(Newberry et al., 2017). Physicochemical fractionation processes can occur
prior to sampling within plants or at the soil–root interface in xerophytic
or halophytic woody plants (Ellsworth and Williams, 2007; Zhao et al.,
2016), or at the time of initial leaf flush (Treydte et al., 2014), but the
effects of those processes are irrelevant to our midsummer sampling in a
humid climate region.

2.4 Data processing and application in seasonal origin index analysis

To compare tree xylem water with precipitation inputs at the respective
sites, seasonal cycles of the δ18O and δ2H of
precipitation were modeled for each site using a sinusoidal isoscape
approach (Allen et al., 2018). Monthly precipitation isotope data were
downloaded from 31 monitoring locations in Switzerland (NAQUA network),
Austria (Austrian Network of Isotopes in Precipitation), and Germany (Global
Network of Isotopes in Precipitation, GNIP); 13 were in Switzerland, and the
remaining 18 were within 135 km of the Swiss border. Sine functions were
fitted to precipitation stable isotope measurements at each monitoring site
using all available data from 2007 through 2015. Parameters describing the
δ18O and δ2H sine functions (offset, amplitude, phase) were then
interpolated across Switzerland by multiple linear regression models using
site latitude, longitude, elevation, mean annual temperature range, and mean
total precipitation amount as the predictors. This yielded a measure of
central tendency (offset) and strength of seasonal cycle (amplitude) for δ18O
and δ2H. For each site, we calculated a typical winter
precipitation value (δwinterP=offset−amplitude) and a typical summer
precipitation value (δsummerP=offset+amplitude). The widths of the
shaded areas in Figs. 2 and S5 show δwinterP and δsummerP±2×RMSE, where RMSE is the root mean square error of
predicted versus fitted amplitude at the precipitation isotope monitoring sites.

To calculate δannP (Eq. 1), we used the volume-weighted mean
precipitation of a 24-month period prior to the xylem water field sampling
campaign (August 2013–July 2015). Monthly values of precipitation δ18O and δ2H at each field site were calculated by
individual-month multiple linear regression models, fitted to monthly
precipitation isotope measurements at the 31 precipitation isotope
monitoring sites, using site latitude, longitude, elevation, mean annual
temperature range, and mean total precipitation amount as predictors (Allen
et al., 2018). In an additional step to account for variations not captured
by the regression model, we kriged the prediction residuals of the
regression model at each precipitation monitoring station, to create monthly
adjustment layers that were added to the regression predictions (Allen et
al., 2018). The mean absolute error in predicting monthly precipitation was
1.2 ‰ δ18O and 9.6 ‰ δ2H. Lastly,
the site-specific precipitation isotope values
were weighted by site-specific monthly precipitation amounts (Meteotest,
Bern, Switzerland) and summed for the 24 months prior to sampling.

To calculate the seasonal origin index, we used
fractionation-compensated lysimeter and xylem water isotope values as
approximations of their source values prior to any evaporative
fractionation. Deviations of soil or xylem water isotope values from local
meteoric water lines (LMWLs) were treated as fractionation effects. To
compensate for these fractionation effects, soil or xylem isotope values
were adjusted back to their respective LMWLs along an evaporation line
slope, calculated using the Craig–Gordon model as implemented for
diffusion-controlled soil evaporation scenarios by Benettin et al. (2018).
Two recent studies have shown that such theory-based approaches are more
robust than evaporation lines fitted to soil water observations, which will
typically be confounded by isotopic variations in precipitation over time
(Benettin et al., 2018; Bowen et al., 2018). Here, we computed the
evaporation line at each site as a function of summer mean relative humidity
and temperature (Fig. S5). Slopes were calculated using both daily maximum
temperatures and daily minimum temperatures to understand the uncertainty
associated with the range of conditions under which evaporation occurs; for
fractionation compensation, we used the mean of the minimum and maximum
temperature slopes. Calculated fractionation slopes across the Swiss sites
were between 2.7 and 3.4 (see Fig. S5), consistent with those reported in a
previous synthesis (Sprenger et al., 2016a). To compensate for the
fractionation effects in xylem water and lysimeter soil water, the monthly
precipitation δ18O and δ2H calculated above for
each site were fitted by orthogonal least squares to generate site-specific
LMWLs. Then, lysimeter and xylem water values were compensated for
evaporative fractionation by shifting them along the site-specific
evaporation slopes estimated above until they intersected the LMWLs. These
intersection points are the fractionation-compensated values used to
represent the precipitation sources of waters in trees and soils (i.e.,
they were used as the δx inputs for the calculation of SOI by
Eq. 1). The effects of this fractionation-compensation step can be seen by
comparing the patterns and species differences in Figs. 2 and S5.

2.5 Statistical analyses

To determine how the δ18O and δ2H ratios of water
in soil and plants varied with soil properties, topography, and climate, we
calculated Spearman (rank) correlation coefficients and Pearson (in Table S1
only) correlation coefficients between site characteristics and both xylem
SOI and lysimeter SOI. Site-mean SOI was calculated for each species and for
shallow (≤30 cm) and deep (>30 cm) lysimeters. To account
for the influence of spatial clustering in correlations, the influence of
any given point was weighted by N−1, where N, calculated by site and by
species, is the number of sites within 10 km that contain the same species;
N was also calculated for lysimeters. Details on the site characteristics
that we examined are provided in Table S1. A stepwise multiple regression
test was also performed to consider interactions between these terms (Table S2).
All statistical tests were performed in MATLAB (Mathworks, Inc.,
Natick, MA, USA).

Tree use of winter-sourced water in midsummer was widespread among our 182
Swiss forest sites (Fig. 2). Low (i.e., winter) SOI values were also
markedly more prevalent in xylem water than in lysimeter samples of soil
water (Fig. 2). Overall, summer (recent) precipitation signatures were
uncommon in these midsummer samples; SOI was >0.5 for only
1 % of oak and beech samples, 5 % of spruce samples, and 5 % of
lysimeter soil water samples (Fig. 3). Winter precipitation signatures were
distinctly more common, particularly in the broadleaf trees (oak and beech);
SOI was <−0.5 for 78 % of oak and beech samples, 17 % of
spruce samples, and 19 % of lysimeter soil water samples. Thus, the
seasonal origin of the broadleaf tree water was distinctly out of phase with
the precipitation at the time of sampling (27 July to 7 August).

Table 1Spearman (rank) correlations between site characteristics and xylem
water and lysimeter soil water SOI, where statistically significant
correlations are indicated with bold fonts (p<0.05) and bold
italic fonts (p<0.01); see Table S1 for a more extensive correlation
table.

Figure 3Distributions of the seasonal origin of water in soils and trees
across Switzerland in midsummer. Beech and oak xylem show a predominance of
winter precipitation. Soil porewaters (sampled by suction lysimeters) and
spruce xylem indicate mixtures of precipitation from multiple seasons.
Seasonal origin index values below 1 reflect waters that are more
isotopically negative than typical winter precipitation (estimated by
sinusoidal fitting of precipitation patterns; see Sect. 2).

The occurrence of winter precipitation in xylem and soils cannot be simply
explained by its carryover in snowpacks or by a lack of summer
precipitation. Beech and oak, which used more winter precipitation than
spruce, occupied lower-elevation (and thus less snowy) sites. Furthermore,
for each species, trees in cooler, snowier sites used less winter
precipitation (see SOI correlations with temperature and snow fraction in
Table 1). Precipitation amounts in Switzerland are distributed relatively
evenly throughout the year (Fig. S1), with approximately 58 % of annual
precipitation falling in warmer months (Fig. S1). This was also true of
2015, when the sampling occurred, despite that summer having slightly less
precipitation than normal (Fig. S1). The average site received 125 mm (and
the driest site received 55 mm) of rain in the 50 days prior to sampling
(Fig. S1). Even if the fractional volumetric field capacity were 0.35
(O'Green, 2012) and infiltration occurred as piston flow, such that summer
precipitation displaced previously stored waters (both conservative
assumptions), 125 mm of summer precipitation would have reached depths of
36 cm (not accounting for evaporation losses). In natural soils, however,
infiltrating waters must percolate deeper than they would by piston flow if
they mix with previously stored soil moisture or partially bypass the matrix
by traveling through macropores (Beven and Germann, 1982; Brooks et al.,
2010; Mueller et al., 2014; Sprenger et al., 2016b). Thus, summer
precipitation could have reached the depths that contained most of the fine
roots (i.e., most roots occur above depths of 15–40 cm, depending on the
site; Fig. 4 and Fig. S2). However, only in the wettest sites did summer
precipitation predominantly contribute to tree xylem water (Fig. 2). While
winter isotopic signatures have previously been observed in summer xylem of
desert plants (Ehleringer et al., 1991), our work demonstrates the
widespread use of winter water in midsummer across diverse humid climates,
prompting the question of how much does tree water use depends on summer
rainfall.

Figure 4Field-measured fine root depths (a) versus site mean annual
precipitation amount and (b) by species for single-species plots. Depth to
root-mass center is the mean rooting depth weighted by density of roots per
horizon. (a) Its variations were not linearly related to mean annual
precipitation (R2<0.01, p=0.40). (b) The box plots show
means (black line) ±1 standard error (medium gray), and ±1 standard
deviation (light gray), of maximum (c) and mean (e) root depths,
for the stands that only contained one of the study species; the data
suggest that the three species have similar average rooting depths across
the sampled sites. See Fig. S2 for more detailed figures on soil and root
depths. These metrics refer to fine roots (see methods).

The study sites (a) share similar ecological communities and forest
management histories and (b) were sampled by a single field crew over a
period of just 12 days, thereby minimizing sampling inconsistencies and
facilitating comparisons of SOI across climatic, topographic, and edaphic
gradients. Total annual precipitation and other climate metrics associated
with water-balance surplus showed statistically significant, positive
correlations with the SOI of xylem water and lysimeter soil water (Table S1). Thus, SOI variations in midsummer xylem water across Switzerland were
not only a product of differences in distributions of species with distinct
rooting habits. The cross-site trends in lysimeter waters mirrored those of
xylem waters (both have stronger winter signatures in drier sites; Fig. 2
and Table 1); this suggests that the xylem water trend does not solely
reflect differences in rooting habits, because there are also trends in
lysimeter soil water seasonal origins. Jasechko et al. (2014) similarly
observed that aquifers were disproportionately winter-sourced in areas with
summer water deficits, probably because under those conditions more summer
precipitation evaporates. SOI was also positively correlated with slope and
elevation, possibly because these topographic variables co-vary with
precipitation (Tables S1 and S2). Surprisingly, correlations between soil
characteristics and SOI were weaker and inconsistent (Tables 1 and S1),
suggesting that soil texture may be less important than climate in their
control over the turnover of plant-available water.

Species differed in their water sources, as reflected by differences in
their xylem water SOI. Spruce is considered to be shallow-rooted, with its
roots mostly occurring in the top 25 cm (Schmid and Kazda, 2002), compared
to beech and oak, whose roots are reported to mostly occur in the top 40 cm
(Schmid and Kazda, 2002; Thomas and Hartmann, 1998). However, measurements
from soil pits excavated in each site show that both maximum and mean
rooting depths were broadly similar among beech, spruce, and oak, with mean
depths usually ranging between 15 and 35 cm and maximum depths usually
ranging between 50 and 120 cm (Fig. 4 and Fig. S2). Regardless of these
observed similarities in rooting depths, the isotope data show that beech
used significantly more winter precipitation than spruce, even within the
same plots (mean SOI difference of 0.53, n=27 sites, p<0.001 by
paired t test, Fig. 5a; similarly, oak SOI was 0.54 lower than spruce SOI in
the one plot where they were paired). In contrast, oak and beech within the
same plots used similar waters (mean SOI difference of 0.10, n=11 sites,
p=0.13 by paired t test; Fig. 5b). In sites with lysimeters, the soil
waters they sampled were significantly less winter-like than beech xylem
waters (mean SOI of −0.20 versus −0.90, n=16 sites, p<0.001
by paired t test; Fig. 5c), suggesting that beech roots accessed water
sources that were deeper (e.g., saprolite) or more tightly held (e.g., fine
pores). These frequently overlooked water sources may be important for
storing winter precipitation and supplying summer transpiration (Rempe and
Dietrich, 2018). In contrast, suction lysimeters generally sample a more
mobile, less tightly held fraction of soil water (Brooks et al., 2010; Sprenger
et al., 2016). Spruce, unlike beech, appear to use water that is more
similar to this more mobile water; the SOI of spruce xylem water was
statistically indistinguishable from lysimeter soil water at paired sites
(mean SOI of −0.27 versus −0.14, n=21 sites, p=0.13 by paired
t test; Fig. 5c). Thus, spruce trees used fundamentally different water
sources than the two broadleaf species, demonstrating niche partitioning in
the rhizosphere across a wide range of soils and climates. Given that the
spruce and beech trees had similar rooting depths but used different source
waters (Fig. 4), we hypothesize that these species niche separations
relate to their relative uptake of water from more vs. less conductive soil
pores.

Figure 5Pairwise comparisons of seasonal origin index values for sites
where (a) spruce and beech are collocated, (b) oak and beech are collocated,
and (c) trees and lysimeters are collocated. The 1:1 lines are plotted for
reference, highlighting that (a) spruce used more summer-sourced water than
beech, whereas (b) beech and oak used similar water supplies. Additionally,
(c) spruce used water similar to lysimeter soil water, unlike beech. Symbols
indicate site means with error bars representing 1 standard error of the
mean, attributable to intra-site variability.

The SOI of xylem water reflects root access to water sources with different
seasonal dynamics, implying vulnerability to different types of droughts.
While deep roots are assumed to mitigate vulnerability to droughts
(Ehleringer et al., 1991; West et al., 2012) by providing access to storages
of past precipitation, this will not be the case where deeper substrates
lack sufficient storage capacity or are not reliably recharged by
infiltrating precipitation (Fan et al., 2017). Here, the data suggest that
SOI variations are not solely a reflection of rooting depth differences
because rooting depths (a) lacked a strong trend across sites (Fig. S2), (b)
were similar among species (Fig. 4), and (c) were weakly correlated with
SOI (Tables 1 and S1). Nonetheless, the xylem water with low SOI directly
reflects access to storages that recharge in winter, regardless of whether
those waters are deeper or more tightly held. Trees that use stored winter
precipitation (e.g., beech and oak in drier regions; Fig. 6) may be less
vulnerable to summer precipitation deficits but more reliant on the soil's
capacity to store sufficient amounts of winter water through the growing
season. As increasing temperatures result in longer growing seasons
(Körner and Basler, 2010), winter water stores may become insufficient
to sustain tree growth in regions with seasonal water deficits (Fig. S3). In
contrast, midsummer xylem water with high SOI (e.g., spruce trees in
central and southern Switzerland; Fig. 6) directly reflects access to
storages with more rapid turnover, in which water from previous seasons has
drained away or been displaced by summer precipitation. Spruce's greater use
of summer rainfall may explain why it is more sensitive than beech to summer
droughts (Brinkmann et al., 2016; Zang et al., 2014). Ultimately, further
research is needed to clarify the extent to which seasonal origin signals
are attributable to rooting habits versus water-transport processes.
Understanding why these spatial and inter-species differences occur and
whether they persist are key to understanding their implications for
predicting forest vulnerability to droughts.

Figure 6Variation in seasonal origins of tree xylem water with mean annual
precipitation (MAP) across Switzerland. Open water and elevations
>2000 m a.s.l. are excluded. In all but the wettest regions, the
seasonal origin index shows a predominance of winter precipitation in tree
xylem.

Beyond these hydrological and ecological insights, our findings have
implications for the use of stable isotopes in climate science and
ecophysiology, because variations in the seasonal origins of xylem water
imply that plant tissue δ2H or δ18O (frequently
used as climatic or ecophysiological proxies) may reflect different seasons
in different species, individuals, sites, and years. The δ2H or
δ18O signatures of plant tissue (e.g., cellulose and leaf
waxes) reflect the initial δ2H or δ18O of the
source water incorporated into plant tissue, as well as climatically and
physiologically controlled fractionation effects (Barbour, 2007). In a
variety of isotope applications, it is often useful to attribute the source
water to summer rainfall (Lawrence and White, 1984) or mean annual
precipitation (Helliker and Richter, 2008); however, we observed waters in
trees that had neither a consistent summer signature nor a consistent mean
annual signature (see Fig. S4). Although these xylem waters were sampled at
one point in time, they document widespread temporal decoupling between
precipitation inputs and plant water uptake. If, as these results suggest,
seasonal origins vary systematically by species and across climatic
gradients, accounting for these variations could aid in interpreting
plant-tissue stable isotopes as environmental proxies.

Variations in SOI also convey information about how soils transport water.
Only the wettest sites clearly demonstrated substantial transport of summer
precipitation to the rhizosphere. Elsewhere, the summer rainfall apparently
did not reach (or potentially bypassed) the relatively shallow depths that
contained most of the fine roots (e.g., 15–40 cm; Fig. 3), suggesting that
infiltration was not a piston-flow process (e.g., translatory flow), as also
previously argued by Brooks et al. (2010). Further evidence for the lack of
translatory flow can be observed in the scarcity of soil waters (sampled by
lysimeters or taken up by roots) with a strong summer signature (Fig. 2);
this indicates that summer precipitation must either mix with or bypass the
storage. Our findings differ from the ecohydrologic separation in a
Mediterranean climate described by Brooks et al. (2010), in which
infiltrating water refilled pore spaces when soils were dry and otherwise
bypassed the rhizosphere when soils were wet. In contrast, our cross-site
comparison indicates that recent precipitation refilled rhizosphere pore
spaces more in wetter sites than in drier sites. The low SOI of xylem water
in beech and oak trees implies that little summer precipitation traveled to
their roots, likely because it either bypassed the soil matrix or was
retained in near-surface soils before quickly returning to the atmosphere
(e.g., by understory transpiration or evaporation of soil water and
intercepted precipitation). A recent study (Brinkmann et al., 2018) focused
on temporal variations in a single site and found that roughly 50 % of
tree water use came from summer precipitation, but this fraction varied
throughout the growing season. Different insights are conveyed by our
snapshot sample from over 900 trees across a network of diverse sites. They
empirically show that the majority of midsummer tree xylem, and by
extension rhizosphere soils, contain only small contributions from summer
precipitation in midsummer. Regardless of these data reflecting a single
snapshot, these measurements imply that the turnover of water (and thus
flushing of solutes) in these trees' rooting zones must be remarkably small
in summer.

Although SOI values do not precisely record water age, the widespread
presence of winter precipitation in summer soils indicates that these waters
often resided in soils for months with minimal mixing, suggesting that
summer precipitation flows preferentially through those soils. We can
explore these flow processes through a back-of-the-envelope calculation.
Mean transit time can be calculated hydrometrically as storage divided by
flux (also often referred to as mean turnover time). We conservatively
estimate the storage above the rooting zone to be 10.5 cm of water (most
roots are above 30 cm, shown in Fig. 3, multiplied by the maximum field
capacity of 0.35). We estimate the mean flux to the roots to be 1.36 mm per
day, calculated as precipitation minus evaporation using the precipitation
(2.51 mm per day) and mean PET (4.6 mm per day) across the sites in the 50 days prior to sampling, and with evaporation assumed to be 0.25 of PET,
which is conservative as an estimated fraction of evaporation over actual
evapotranspiration in full-canopy forest (Schlesinger and Jasechko, 2014).
Assuming steady-state conditions, this yields an estimated hydrometric mean
transit time (or turnover time) of 77 days in summer (and it must be
substantially shorter in spring or winter because PET is lower and
precipitation rates are similar). If mean transit times are 77 days and soil
water storages are composed of waters with undiluted, midwinter
precipitation isotope values, then stored waters must be substantially older
than the mean transit time. This contrast between storage ages and the mean
transit time would suggest that soil water flows are neither well-mixed nor
translatory and instead are preferential (sensu Berghuijs and Kirchner, 2017),
although it remains unclear how fully roots sample and reflect the age of
soil water storage. Indeed, these root surveys are coarse characterizations,
and some deeper (and potentially overlooked) roots could transport a
disproportionately large fraction of water. Regardless, the empirical
insights shown here – specifically, trends in the seasonal origins of water
in soils across climates, and differences in the use of recent precipitation
versus older stored precipitation among species – may find application in
better parameterizations of plant uptake of water from dynamic storages in
gridded hydrological or ecological models.

More broadly, the analytical framework introduced here provides a new tool
for applying stable isotope data to explore a wide variety of ecological and
hydrological processes. Here, the seasonal origin analysis aids in
describing plant–soil-water interactions and how they vary across
landscapes; specifically, examining the seasonal origin of tree water
revealed (1) the consistent inter-species differences in rhizosphere water
niches, (2) the long residence times of root-zone soil moisture in summer,
and (3) the need to consider the overexpression of different seasons'
precipitation when interpreting plant-tissue isotopes. We suspect that
insights may also be revealed through applying this seasonal origin index
analysis to groundwater (sensu Jasechko et al., 2014), stream water, or even
plant and animal tissues.

The seasonal origins of precipitation used by trees, which reflect the
interplay between infiltration dynamics and root distributions, have not
previously been systematically investigated. We used a spatially extensive
snapshot sample of xylem water from Swiss forest plots to quantify the
seasonal origins of water used by trees in midsummer. Xylem waters in 918
trees from 182 sites (and soil lysimeter water from a subset of these sites)
were sampled and analyzed for δ18O and δ2H by a
single team using consistent methods (thereby avoiding many uncertainties
that are common to meta-analyses). By applying a new index that
characterizes the occurrence of summer versus winter precipitation in these
xylem samples, we show that trees mostly used winter precipitation in
midsummer in all but the wettest regions of Switzerland (Fig. 1). Summer
precipitation isotope signatures were uncommon in shallow soils, deep soils,
and tree xylem (Fig. 2), suggesting that infiltrating precipitation does
not simply displace stored soil waters. There was consistent partitioning in
the water sources used by different species (Fig. 3): beech and oak
almost entirely used winter precipitation, whereas spruce used more mixed
sources that were isotopically similar to the water extracted by suction
lysimeters (presumably from more conductive pores). The widespread
prevalence of winter precipitation in midsummer tree xylem suggests that
(a) the turnover of water (and thus flushing of solutes) in these trees'
rooting zones must be remarkably small in summer and (b) plant-tissue
isotope proxies may not consistently capture summer climate signals. These
findings conflict with common assumptions on tree water use and provide
empirical support for developing more realistic representations of
root–soil-water interactions.

The data that support the findings of this study are
available in the Supplement. We acknowledge data contributions
by International Atomic Energy Association and GNIP contributors as well as
Swiss, German, and Austrian federal monitoring agencies (Swiss Federal Office
of the Environment (FOEN) and its NAQUA program, MeteoSwiss, Austrian Network
of Isotopes in Precipitation, Austrian Zentralanstalt für Meteorologie
und Geodynamik, and Deutscher Wetterdienst).

STA conceived and executed the analysis, with input from GRG
and JWK. STA, JWK, and GRG wrote the paper. SB, GGR, and RTWS initiated the project
and coordinated the 2015 field and lab work. SB leads the long-term
measurement network.

We thank Nadine Engbersen, Clara Romero, Lola Schmid, and
the Institute for Applied Plant Biology team for assistance with sample
collection and processing; Wouter Berghuijs and Julia Knapp for comments on
the manuscript; and Wouter Berghuijs and Paolo Benettin for many useful
discussions during the writing process. We thank two anonymous reviewers and
Editor Markus Weiler for their helpful feedback. The forest departments of
the cantons AG, BE, BL, BS, GR, SO, TG, ZH, and ZG, as well as the
environmental offices of Central Switzerland, funded the tree sampling. This
project was funded by a Swiss Federal Office of the Environment agreement
with Gregory R. Goldsmith and James W. Kirchner. Gregory R. Goldsmith was supported by
funding from the European Commission's Seventh Framework Programme
(FP7/2007–2013) under grant agreement number 290605 (COFUND: PSI-FELLOW)
while at the Paul Scherrer Institute.

Brinkmann, N., Seeger, S., Weiler, M., Buchmann, N., Eugster, W., and Kahmen,
A.: Employing stable isotopes to determine the residence times of soil water
and the temporal origin of water taken up by Fagus sylvatica and Picea abies
in a temperate forest, New Phytol., 219, 1300–1313, https://doi.org/10.1111/nph.15255,
2018.

We used stable isotopes of xylem water to study differences in the seasonal origin of water in more than 900 individual trees from three dominant species in 182 Swiss forested sites. We discovered that midsummer transpiration was mostly supplied by winter precipitation across diverse humid climates. Our findings provide new insights into tree vulnerability to droughts, transport of water (and thus solutes) in soils, and the climatic information conveyed by plant-tissue isotopes.

We used stable isotopes of xylem water to study differences in the seasonal origin of water in...